Grouting for Prestressed Concrete Box Girder
The grouting of the prestressed concrete box girder shall meet the following requirements:
- The contractor shall use vacuum-assisted grouting technology to grout the tunnels, and the slurry shall be mixed with vacuum grouting additives and rust inhibitors, and the dosage shall be determined by experiments.
- The cement used should be accurately weighed, and the production period should not exceed one month.
- The water-cement ratio of cement slurry should be controlled within 0.30~0.35. The bleeding rate should be controlled at 2%, and the bleeding should be absorbed by the slurry within 24 hours. The fluidity of the slurry should be controlled within 14~18s, the swelling rate of the slurry should be <5%, the initial setting time should be more than 3h, the final setting time should be less than 24h, and the slurry temperature should not exceed 35ºC during grouting.
- In order to achieve the required characteristics of the cement slurry, chemical additives can be added. Additives should have the effects of reducing water, retarding, micro-swelling, and increasing the workability of the slurry, but must not contain substances that damage the prestressed tendons and anchorages, especially corrosive media such as chloride and calcium nitrate. The expansion component in the additive is strictly prohibited from containing aluminum powder.
- The ratio test of the slurry material mixture and the slurry performance test should be in accordance with the “Technical Specifications for the Application of Concrete Admixtures” (GB50119-2003) and the “Technical Specifications for the Construction of Highway Bridge and Culvert”(JTG/T 3650-2020). The detection method and performance of vacuum grouting additives should meet the requirements of “Concrete Admixtures” (GB/T8076-2008) and “Expansion Agents for Concrete” (JC476-2001), and the test results should be submitted to the supervision engineer for approval.
- The mortar blender should use a high-speed mixer with a rotation speed of 1300r/min. The mortar pump should be able to grout continuously and evenly and can perform constant pressure work above 0.7Mpa.
- The pump of mortar pump and its suction cycle should be completely sealed to prevent air bubbles from entering the cement slurry. It should be able to maintain the pressure on the squeezed pipe and be equipped with a nozzle. The nozzle should be able to keep the pressure in the pipe without loss, and it should be equipped with an emergency unloading function.
- The pressure gauge should be calibrated before the first use. All equipment should be thoroughly cleaned with clean water at least every 3 hours during the grouting operation, and should also be cleaned once every day at the end of use.
- Vacuum pump: water ring vacuum pump is used, tap water is used for circulating water, and the water temperature cannot exceed 40°C. The vacuuming capacity should be greater than 90% (-0.09Mpa), and the vacuuming efficiency should not be less than 40m³/h.
- After the tensioning is completed, the ducts must be blown dry with high-pressure air without water vapor, then the prestressed anchorages shall be sealed, vacuumed, and grouted. The volume of the mortar blender storage tank must be greater than the volume of the prestressed duct to be grouted.
- The strength of cement slurry should meet the design regulations. When there is no specific design requirement, it should not be less than 80% of the concrete strength of the main structure.
- In addition to the conventional slurry ratio experiment before grouting, the inclined tube test, the density test, and the simulated grouting test should also be carried out.
- When grouting, each working group should keep no less than 3 sets of samples (each group is 3 pieces of 70.7mm×70.7mm×70.7mm cube samples), standard curing for 28 days, check their compressive strength, as a basis for evaluating the quality of cement slurry.
- When the air temperature or cabinet temperature is lower than 5℃, grouting is not allowed. The temperature of the cement slurry shall not exceed 32°C.
- Duct grouting should be carried out as soon as possible after the completion of the prestressed tension, generally not more than 3 days. When grouting, the slurry is pressed into the duct from the grouting end, and discharged from the vacuum end until the consistency of the outflow reaches the consistency of the injection. The duct should be filled with slurry. The duct grouting of a simply supported beam should be injected from one end of the beam and flow out at the other end. The consistency of the outflow must reach the specified consistency.
- The duration of cement slurry from preparing to pressing into the duct should not exceed 40 minutes. The cement slurry should be kept flowing before use and during grouting.
- According to the vacuum-assisted grouting process, when the slurry flows from the vacuum end of the duct, a waste slurry removal operation should be carried out at both ends of the duct, and then the pressure is maintained at 0.7MPa for no less than 2min. The compressed duct should be protected from vibration within one day. Within 48 hours after injection of cement slurry in the duct, the temperature of concrete shall not be lower than 5°C, otherwise, heat preservation measures shall be taken. When the temperature during the day is higher than 35℃, grouting should be carried out at night. Two days after grouting, check the cement slurry density at the injection end and vent hole, and deal with it if necessary.
- The contractor shall have a complete grouting record, including grouting date, water-cement ratio, admixtures, grouting pressure, test block strength, details of obstacle accidents, and work to be done. Copies of these records should be sent to the supervision engineer within 3 days after grouting.
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